Oriented silicon steel sheets are essential material for manufacturing various transformers. A transformer consumes electric power because of its iron loss. For this reason, to reduce iron loss is an ever-seeking target in production of oriented silicon steel sheets. Iron loss of silicon steel sheets is divided into hysteresis loss and eddy current loss. The eddy current loss is subdivided into classical eddy current loss (caused by eddy current) and abnormal eddy current loss (caused by magnetic domain wall movement and proportional to width of magnetic domain). Under the condition of power frequency, abnormal eddy current loss amounts is nearly to half of iron loss. With the development of metallurgic technology, metallographic texture (110) [001] has been constantly getting to perfect, it will certainly cause crystal grains to grow up too quickly during secondary re-crystallization and to become big grains, this will cause magnetic domain width to enlarge and the proportion of abnormal eddy current loss to increase. In this case, it gets more and more difficult to continuously use conventional metallurgic process to further reduce iron loss.
In order to further reduce iron loss of oriented silicon steel sheets by fining magnetic domain, many steelworks and steel research institutes in the world have carried on research on technology for fining magnetic domain and have successively developed various surface treatment technologies, such as machining method, laser irradiating method, discharge treatment method, plasma jet irradiation method, local heating method, ultrasonic vibration method, fluid jetting method, and so on. These methods can fine magnetic domain by fining the space between 180° magnetic domain walls, so as to attain the goal of reducing iron loss. Among them, the laser scoring method is the most prominent.
By reducing major magnetic domain width of oriented silicon steel, the laser irradiating method reduces eddy current loss of the sheet. In more detail, by using heat of laser beam, this method enables silicon steel sheet to generate an elastic-plastic deformed area beneath its surfaces and reduces major magnetic domain width of oriented silicon steel by pressure stress generated in the elastic-plastic deformed area and tension stress between the scored lines, so as to attain the goal to reduce eddy current loss. Being able to reduce iron loss greatly and being a kind of non-contact machining, this method has very high reliability and controllability.
There have been many patents which relate to the method to reduce iron loss of oriented silicon steel by laser scoring.
The Chinese patent CN1216072A disclosed a grain-oriented electric steel sheet with excellent magnetic property and its production process and equipment. In this process, pulse laser irradiating is used to decrease the space between 180° magnetic domain walls and so to improve magnetic property. This process uses Q-switch CO2 laser to irradiate the surface of grain-oriented electric steel sheet with irradiating power density adjusted to be not above a threshold which will damage the insulation film on surfaces of the steel sheet, by using an oval facula whose major axis is in transverse direction of the steel sheet to irradiate the surface of the steel sheet and by making the oval facula partially overlap so as to generate a continuous laser beam and to provide sufficient accumulated irradiation energy necessary for improving magnetic property. Thereby, the problem that conventional pulse laser irradiation technique trends to damage the insulation film on steel sheet surfaces is solved rather successfully. In that process, however, scanning speed can not be too fast in order to ensure sufficient accumulated energy, which restricts scoring rate of a laser scoring machine and can not meet requirements for high-speed laser scoring.
The Chinese patent CN1761764A disclosed a sort of oriented electromagnetic steel sheet with good magnetic property and its manufacture process. In that process, scanning irradiation by continuously oscillating optical fiber laser beam with wave length λ of 1.07≦λ≦2.10 μm is carried on to reduce iron loss of oriented silicon steel. However, to use continuously oscillating optical fiber laser beam with wave length λ of 1.07≦λ≦2.10 μm leads to a rather high beam intensity nearby the center, which is likely to give rise to unsmoothness of the surface to have been irradiated. If it is intended to solve this problem by reducing mean output power P of the laser device, it is necessary to reduce scanning speed simultaneously, which will certainly impair machining rate of the laser device.
The Chinese patent CN 101209514 disclosed an on-line laser high-speed scoring device based on an array of vibrating mirrors. The device includes support frame, carrying belt, vibrating mirror array, laser array and controls for controlling the vibrating mirror array and laser array. Because of incorporating the vibrating mirror array, this method is able to efficiently raise laser scoring speed and has advantages such as flexible adaptability, high scoring speed, wide scanning breadth, high production efficiency, but regrettably, that method can not simultaneously conduct scoring on both surfaces of a steel strip. In comparison to other laser scoring methods, this method is not superior.
The Chinese patent CN1076492A disclosed a laser treatment process and its device for reducing iron loss of silicon steel sheet. By using the characteristics of laser to heat and cool both quickly, the process makes score-formed treatment on oriented silicon steel sheet and promotes heated area to generate minor plastic deformation and high-density dislocation so as to form sub-boundary of grains and thus to decrease wall length of major magnetic domain, meanwhile to cause residual tension stress so as to attain the goal of fining magnetic domain and reducing iron loss. The process is feasible and the device is simple. However, in that process, iron loss can be reduced by only about 5% and machining rate is not taken into account.
Moreover, the Chinese patent CN101348853 disclosed a process of reducing iron loss of common orientated electric steel. In that process, surface coating film of silicon steel sheet will be destroyed badly and need to be re-applied after being scored. In addition, inconvenient operation and high production cost has had this process to have been out of use. The Chinese patent CN1244597A disclosed a process of treating surface of heat-resistance oriented electric steel sheet by a laser. By making local areas of silicon steel sheet alloyed, this process is able to optimize magnetic domain distribution and so to reduce iron loss, thereby to make electric steel sheet to have stability in high temperature and excellent aging property. However, in that process, it is necessary to strip off the insulation coating film of silicon steel sheet prior to implementing the process, and the silicon steel sheet needs to be re-applied with a insulation coating film after being scored. The process is too complicated to implement and not applicable for mass production.